关系数据库的任何操作都是由3种基本操作组合而成，这3种基本操作不包括______。
A．连接
B．索引
C．选择
D．投影
参考答案：B
Those who ____ in ____ compositions, please hand them in this afternoon. [ ]
A. hasn't handed; his
B. haven't handed; their
C. has handed; their
D. have handed; his
毕奥呼吸常见于（）。
A.急性喉炎
B.呼吸中枢功能障碍
C.情绪激动
D.右心功能不全
E.左心功能不全
